Electric motors are used in many, many applications. They generally operate by harnessing magnetism to rotate an output shaft in response to electricity. Typically, this is done by providing one or more wires (i.e., "windings") with electric power, which are configured within the motor in a way that causes the rotation.
No matter the application, a common danger to motor life is motor overheating. This can be caused by jamming or stalling of the motor, where its output shaft is prevented from freely-rotating. While some amount of loading on the motor's output shaft always occurs, significantly so when the motor is performing heavy work, if the loading is large enough, the windings can be caused to catastrophically overheat and damage the motor. Thus, no matter the type of product or machinery, if it has a motor, motor overheating can be a significant limit to product's or machinery's useful life.
"Cargo Power Drive Units," or "cargo PDUs," are one particular application of an electric motor, and are located within the belly of an aircraft. In PDU applications, cargo is normally supported by a system of freely rotating bearings, as shown in FIGS. 1 and 2. This bearing structure that enables the cargo to be easily pushed within the aircraft to its intended position. Individual PDUs are mounted within the floor of an aircraft cargo bay, and each has its own motor and rubberized roller. When turned on, sets or banks of individual PDUs are commonly elevated from beneath the floor to just above the bearings, to propel cargo while it is supported by the bearings. This enables the cargo to be mechanically positioned by a single operator, without requiring extensive manual labor.
Each of several sets of PDUs are arranged along a path of conveyance, and each set consists of several aligned PDUs which are driven in common. That is, an operator manipulating the PDU controls (which consist essentially of an on/off switch and a joy stick) will cause all of the PDUs in a given set to lift and rotate in the same direction at the same time. This enables the operator to simultaneously move more than one piece of cargo within the aircraft. In the system shown in FIGS. 1-2, there are five sets of cargo PDUs: Front section left and right track sets 13, 15, rear (kink) section left and right track sets 17, 19, and a lateral set 21 positioned at the cargo bay door 23 that connects left and right tracks at the front/rear section junction.
The problem of motor overheating, discussed just above, can be a serious limitation on the effective life of PDUs. For example, as there are typically numerous pieces of cargo to be stowed within the aircraft, several pieces are typically simultaneously moved by one energized set of cargo PDUs. However, a first piece of cargo reaching the end of the aircraft will be blocked, and unable to move further, while the entire set of energized cargo PDUs continues to rotate in elevated condition and propel the remainder of the cargo along the length of the aircraft. The rollers underneath stopped cargo thereby abrasively "scrub" the bottom of the cargo, if it is light, and causes damage to the cargo and wear to the PDU wheel or roller. In the case of heavier cargo, this jamming can also cause the PDU motors to overheat and burn out.
The problem of motor overheating is by no means limited to the environment of PDUs; it is significant generally whenever motors are used.
Solutions to motor overheating have traditionally relied upon use of either a bi-metal switch or a fuse. These elements are placed in line with the motor's supply of electricity, and are triggered when the amount of current drawn by the motor increases above a predetermined amount. Use of these elements alone does not provide an ideal solution to all motor overheating problems, however.
For example, fuses and bi-metal switches have serious life and repeatability problems. It is generally necessary to replace the fuse or reset the bi-metal switch each time they are triggered, which can be labor intensive, particularly so where multiple switches or fuses are utilized. Frequent resetting of bi-metal switches imposes a limited life on the switch and subjects it to progressive deterioration.
Also, there is usually a time lag between an increase in motor temperature and the cutting-off of motor power by the bi-metal switch or fuse, and these elements do not always function correctly. Under conditions of jamming, where motor temperature can rise very quickly, motors still can experience damage.
Further still, the use of fuses and bi-metal switches do not provide flexibility, because they only are affected by current, which is not necessarily the same thing as motor temperature. For example, in applications where a three-phase motor is used, currents of three different phases are supplied to supplied to three different windings of the same motor. In such systems, motor faults besides jamming can cause overheating, but these faults sometimes cannot readily be distinguished from jamming using bi-metal switches and fuses; bi-metal switches and fuses are triggered by a rise in current fed to motor, but not by other changes in current consumption that might reflect motor faults other than severe strain. It would be useful to have a motor control system that distinguishes a system malfunction, such as degradation of one of the windings, from a jammed, overheating motor. This would permit quick replacement of a malfunctioning motor.
There has existed a definite need for a motor control system that instantaneously detects motor overheating and turns off the supply of power to the motor. What is needed is a system that can be repeatedly used without degradation, thereby further improving the effective life of the system. Further, there has also existed a need for a system that can detect motor jamming and distinguish jamming from other types of motor faults. The present invention solves these needs and provides further, related advantages.